This invention relates to the continuous casting of thin steel strip in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls forming a nip between them. The casting rolls are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be received from a ladle through a metal delivery system comprising a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. The casting pool is usually confined between refractory side dams held in sliding engagement with the end surfaces of the casting rolls so as to restrict the two ends of the casting pool against outflow. The atmosphere in the casting area, or chamber, above the molten metal in the casting pool is usually controlled by delivering an inert gas such as argon or nitrogen to the area above the casting pool.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at temperatures in the order of 1400° C. or above. An enclosure is provided beneath the casting rolls to receive the hot cast strip, through which the strip passes away from the strip caster in an atmosphere that inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by delivering a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, in the enclosure beneath the casting rolls. Alternatively, or additionally, the enclosure may be substantially sealed against ingress of an ambient oxygen-containing atmosphere during operation of the strip caster, and the oxygen content of the atmosphere within the enclosure may be reduced by oxidation of the strip to remove oxygen from the enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
During operation, the metal flow rate and molten metal temperature are controlled to reduce the formation of solidified steel skulls in the casting pool in the area where the side dams, casting rolls and meniscus of the casting pool intersect, i.e. the “triple point” region. These unwanted solidified steel skulls, may form from time to time near the side dam and adjacent the end of the delivery nozzle, and can cause defects to the cast strip known as “snake eggs.” When these skulls go through the roll nip, they may also cause the two solidifying shells at the casting roll nip to “swallow” additional liquid metal between the shells or may cause the strip to reheat and break disrupting the continuous production of coiled strip. The snake eggs defects may also be detected as visible bright bands across the width of the cast strip, as well as by spikes in the lateral force exerted on the casting rolls as they pass through the roll nip. Such resistive forces are exerted against the casting rolls in addition to the forces generated by the ferrostatic head in the casting pool. Additionally, skulls resulting in snake eggs in the cast strip passing through the nip between the casting rolls can cause lateral movement of the casting rolls and the side dams. To resist the increased forces generated, bias forces have also been applied to the side dams, increasing the force the side dams exert on the ends of the casting rolls, and, in turn, increasing side dam wear. There remains, therefore, a need to control the formation of unwanted solidified skulls in the casting pool and formation of snake eggs in the thin metal strip.
The thickness of the cast strip at any localized point across the width of the strip is dependent upon the thickness of the two solidifying shells on the opposing casting surfaces of the casting rolls, and the amount of liquid or mushy material that is passed through the nip between the two solidifying shells. An excess amount of liquid, or mushy, material between the shells will cause a localized expansion of the strip causing ridges to form in the strip surface. The amount of mushy that passes through the nip at any localized point across the width of the casting rolls is dependent upon the localized forces exerted on the forming strip at that point.
The force distribution across the casting rolls exerted on the strip is dependent on several factors. Such factors include: the cold profile of the casting rolls; the contour of the rolls due to bulk heat flux; the contour of the casting rolls due to heat flux distribution across the casting roll; and, the thickness of the solidifying shells at any point across the casting rolls. The cold profile of the casting rolls is the contour the casting rolls have before being placed in the continuous caster. Bulk heat flux refers to the rate of heat transfer from the molten metal pool into the casting rolls across the entire length of the casting rolls. The heat from the molten metal in the casting pool causes the outer portions near the casting surfaces of the casting rolls to expand forming a concave profile along the casting rolls.
To control this change in the casting roll profile during operation, the casting rolls have been generally formed having a concave profile smaller in circumference such that when the outer portions of the casting rolls heat and expand in operation the rolls have a desired contour. The two solidifying shells on the surface of the casting rolls coming together at the nip exert an outward force on the casting rolls as they pass through the nip. The force exerted by the shells on the casting rolls corresponds to the size of the shells passing through the nip and the amount of mushy or liquid material between the shells. There is, therefore, a need for a method of continuously casting metal strip which allows for the control of force distribution across the length of the casting rolls and the thickness of the cast strip, and in particular a method of controlling the force exerted upon the forming strip at localized points along the casting roll.
The concave casting roll comprises an outer sleeve, generally made of copper or copper alloy. Attenuation of the copper sleeve has been observed, where the temperature gradient of the casting rolls shows attenuation adjacent the end portions of the casting rolls. Tests demonstrate that the temperature profile of the crown in the surface of a casting roll over at least the last 150 mm from the end portions increases compared to the center portions. The end portion of the copper sleeve constrains the lateral expansion of the center portions of the copper sleeve in the axial direction of the casting roll and the observed attenuation in the copper sleeve end portions has the effect of increasing this constraint on the cylindrical tube of the casting roll, increasing diameters in the central section of the casting roll, and thus causing the casting roll to “belly out” or “crown up” more. This results in a corresponding decrease in the strip cross-sectional profile due to the increased roll crown. There is therefore, presently a need to locally affect this attenuation observed within 300 mm or 150 mm of the casting roll end portions.
Presently disclosed is a method of continuously casting metal strip that comprises the steps of assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between casting rolls through which thin cast strip can be cast, assembling a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the ends of the casting rolls, the casting pool forming a meniscus with each casting surface of the casting rolls, and counter rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly from the nip. The method also includes delivering a shell thickness controlling gas to select areas within 300 mm of end portions of at least one casting roll downwardly toward the meniscus between the casting pool and the casting surface of the casting roll selected to control thickness of the metal shell, sensing the temperature and/or thickness profiles of the cast strip downstream from the nip to determine high or low temperature areas of the cast strip, or thick or thin strip thickness profile areas of the cast strip, within 300 mm of the end portions, and causing the gas to be delivered to the high or low temperature areas, or thick or thin strip thickness profile areas, to change the localized thickness of the metal shell.
Also disclosed is a method of continuously casting metal strip that comprises the steps of assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between casting rolls through which thin cast strip can be cast, assembling a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the ends of the casting rolls the casting pool forming a meniscus with each casting surface of the casting rolls and counter rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly from the nip. The method further includes delivering a shell thickness controlling gas to select areas within 300 mm of end portions of at least one casting roll downwardly toward the meniscus between the casting pool and the casting surface of the casting roll adapted to control localized thickness of the cast strip.
In any embodiment of the method of continuously casting metal strip may comprise where the gas is delivered to the meniscus at a position between 30 mm and 50 mm from the end portions of the casting rolls. Further, or alternatively, the method may comprise where in addition determining high or low temperature areas of the cast strip within 50 mm, or within 150 mm, of the end portions and causing the gas to be delivered to such high or low temperature areas to change the thickness of the metal shell.
In other alternatives, the method of continuously casting metal strip may comprise where the gas is delivered to the meniscus from a distance less than 150 mm above the casting pool.
In further alternatives, the method of continuously casting metal strip may comprise where in addition the gas is delivered to the meniscus near the end portions of the casting roll, and, in still further alternatives, the gas is delivered to the meniscus at a second position between 50 mm and 300 mm from the end portions of the casting roll. In any alternative, the gas may be delivered to the meniscus at a first position within 50 mm from the end portions of the casting roll and a second position between 50 mm and 300 mm from the end portions of the casting roll.
The gas may be delivered to the meniscus of both casting rolls within 300 mm from the end portions of each casting roll.
In any alternative, the method of continuously casting metal strip may comprise where the shell thickness controlling gas is selected from the group consisting of argon, carbon dioxide, hydrogen, helium, nitrogen, air, dry air, water vapor, carbon monoxide and mixtures of thereof.
Further, the method may comprise assembling a carbon seal laterally positioned above each casting roll to restrict oxygen from entering the casting pool.
Various aspects of the invention will become apparent to those skilled in the art from the following detailed description, drawings, and claims.